[backport] CI refactor to v2.1.x branch

[davidcassany]

This allows to run PRs to v2.1.x branch following the same upstream workflow setup, using a local registry and a github cache

This backport required several additional related changes that are not strictly exclusive to the CI workflow refactor. I added all them together in this PR as this way I can verify tests are passing green. Here is an explanatory list of whats included:

• 2061 (This is required to enable adding authorized SSH keys in ISOs, bugfix)

• 8876278f9f4b8396b4e6ecb644701dd920f56f95 (gets rid of the old cos password)
• bf6a01ad129f3765d84e80092ae33bdde809ae92 (allows password less connections in tests using an SSH key pair)
• 76159d9f34bf907d26fddf2de9533f2709d17184 (keeps using legacy cos passwords for upgrade test which uses and old image to upgrade from)

• 2131 (required to allow CI tests to pull images from an local insecure registry without TLS)

All other four commits are the backport of the CI refactor itself.

[davidcassany]

This PR will show some failures, this is because the old worlflow was configured to run the workflow of the target branch while the new one runs the workflow defined in the branch itself. As a result this PR runs two workflows:

• one based on the target branch which will fail because it uses the old workflow (trying to make use of the ghcr.io registry to share images across jobs and VMs) against the current Makefile from this PR
• one based entirely on the current PR which must pass as this uses the GHA cache and a local registry to share containers with launched VMs.